I dont know. The 104 has the distance estimator, and the 420 has the separate rangefinder thing. I've already converted the 104 to two AAA batt's, and I've already put a pack of film through it. I can't seem to find any light leaks, it seems to function properly, and its rather clean. There isn't any rust in the film compartment, and the bellows on the 104 seem to be a little more supple than the 420. The bellows on the 420 seem to be in good condition, but they feel a little more stiff than the 104, maybe its because it's been stored for a while.

I dont think I'm interested in any frankenstein type projects because I'd most certainly ruin it if I tried cutting and pasting lenses from one camera to the other.

I'm assuming by 'magic eye' you are referring to the rangefinder focusing thing? Or are you referring to the sensor on the front that sets your shutter speed?